Monopropellant and method of using same



United States Patent 3,236,046 MONOPROPELLANT AND METHOD OF USING SAME Homer B. Wellman, Pacoima, Califl, assiguor to Sundstrand Corporation, a corporation of Illinois No Drawing. Filed Feb. 11, 1960, Ser. No. 8,192 37 Claims. (Cl. 6035.4)

This invention relates to a method of operating a gas generator with a monopropellant while substantially eliminating coke deposition and to a monopropellant composition capable of substantially coke free decomposition in a gas generator.

A major difiiculty is encountered when ethylene oxide is used as a monopropellant for the production of energy in a gas generator as a result of auto-decomposition, cracking or nonoxidizing exothermic breakdown. This major difliculty is the formation of carbon or coke in the decomposition reaction apparatus and particularly in the exhaust nozzle of the apparatus. Heavy deposition of carbon and coke in the reaction apparatus clogs the Working parts of the machinery, and limits useful operating life of the apparatus between cleansings. No satisfactory method has ever been suggested to control the deposition of carbon or coke in such gas generating apparatus.

It is an object of this invention to provide a method for operating a gas generator with a monopropellant while substantially eliminating coke deposition.

It is another object of this invention to provide a liquid monopropellant capable of substantially coke free decomposition in a pretreated stainless steel gas generator.

It is still another object of this invention to provide a monopropellant having improved ignition capabilities when used in a pretreated stainless steel gas generator.

It is yet another object of this invention to provide a monopropellant capable of use at increased propellant temperatures without coking in a pretreated stainless steel gas generator.

It is still another object of this invention to provide a method of operating a stainless steel gas generator with a monopropellant and Without coke deposition which comprises the steps of passivating the interior surfaces of the generator at elevated temperatures with sulfurous materials and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing by weight about .2 to .8% sulfur in the form of a suitable sulfur bearing additive.

It is yet another object of this invention to provide a liquid monopropellant capable of substantially coke free decomposition in a pretreated stainless steel gas generator comprising ethylene oxide containing by weight from about .2 to .8% sulfur in the form of a suitable sulfur bearing additive.

Other objects and advantages of this invention will become apparent from the following description.

The purpose of this invention is to enable improved operation of hot gas generators with ethylene oxide monopropellant over a wide ambient temperature range without the undesirable side effect of coke deposition. A prime cause of malfunction with ethylene oxide is the deposition of coke in the gas generator exhaust nozzle with consequent decrease in flow rate of hot gases and decrease in thrust obtained.

The initial deposition of coke on the surfaces of the gas generator may be largely overcome by methods of treatment which will be more clearly explained later.

However, this in itself is not enough and subsequent decomposition of ethylene oxide as a monopropellant in such a treated gas generator would still result in the deposition of coke on the surfaces of the generator. For this reason it is necessary that the monopropellant contain a concentration of sulfur in the form of a suitable sulfur compound which will continue to prevent the deposition of coke on the surfaces of the generator.

Several 300 series stainless steels were tested in gas generators. For example, liners made of 302, 304L, 316L, 321, 347 and 410 stainless steel have been used. Of these, type 347 proved particularly desirable because of its higher stability against sensitization and lower rate of sulfide attack.

A sulfiding pretreatment of the steel surfaces of the gas generator, which are generally stainless steel is one of the ways of preventing initial coking at a high characteristic length or L*. The characteristic length or L* is directly proportional to the stay time of the reactants and is defined as the ratio of gas generator volume to exhaust nozzle area. Without pretreatment such as sulfiding of the generator, operation is confined to a narrow L* range between a minimum required for stable combustion and a low maximum where severe coking and exhaust nozzle fouling commences. Operation in this narrow L* range is, of course, unsatisfactory and does not provide the desirable or necessary flow rate of hot gases and thrust.

A pretreatment with hydrogen sulfide diluted with an inert gas such as nitrogen at temperatures from about 1500 to 1700 F. for about two hours will fully passivate a gas generator. The conditions of pretreatment may be varied widely within this temperature range. For example, at 1500 to 1700" F. a gas velocity of 0.4 ft./sec. with a 0.4% H 8 concentration may be used. In the same temperature range of 1500 to 1700 F. the concentration of H S may be raised to 5% and the gas velocity decreased to about 0.01 ft./sec. Pretreatment above 1700 F. with hydrogen sulfide may be effective but is detrimental to the stainless steel substrate on account of excessive intergranular attack by the sulfur. Pretreatment below 1500 F. with hydrogen sulfide is hardly effective for passivation although a deep and even attack by sulfur is readily obtained. Passivation may be defined as a pretreatment which substantially reduces initial catalytic coke formation.

In one example of this type of pretreatment a two hour treatment with 5% hydrogen sulfide was made with the reactor maintained at the desired temperature in an electric muflie furnace. Oxygen-free nitrogen was first flowed through the reactor until the desired temperature of 1700 F. was obtained. A total gas flow rate of ml. per minute for two hours was then maintained with hydrogen sulfide diluted to 5% with nitrogen. This flow was continued while subsequently cooling the reactor.

Another method of passivating the steel surfaces of the gas generator is to operate it for about two hours at low L*, i.e. below 2000 inches and preferably near 1000 inches, with a suitable concentration of a sulfur compound in the ethylene. oxide monopropellant. For example, passivation was readily obtained by a series of decompositions at low L*, for example 1710 inches, of ethylene oxide containing up to 0.8% sulfur content in sulfur compounds for 1 /2 to 2 hours. For example, four twenty-four minute runs using ethylene oxide and 1. 6% by weight dirnethyl sulfide were found sufiicient. About 6% dioxane may also be included in this composition to'lower the flame temperature and thus decrease the coke deposition as will be explained later.

The stainless steel surfaces of the gas generator may also be passivated by heating them in a moist argon atmosphere for two hours at a temperature of about 1500 F. to 1700 F. For example, a two hour treatment of a 302 stainless steel liner with 0.5% by weight of moisture in the argon produced a thin, adherent, black oxide film on the metal surfaces. This treatment resulted in low coke formation in the gas generator which was operated with an ethylene oxide monopropellant containing an amount of sulfur bearing additive. For this treatment, nitrogen, helium or other inert gas may be substituted for argon, and oxygen for the water vapor. A low concentration of moisture or of oxygen in the range from about 0.2 to 2.0% by weight may be used depending on the total gas flow rate and temperature. Temperatures above 1700 F. are undesirable on account of excessive intergranular attack of the metal which leads to structural failure. Nitrogen, helium, argon or other inert gas is equally suitable either for H 8 dilution or for oxygen or moisture dilution.

Grit blasting has also been proven to be an effective method of treating the metal surfaces within a gas generator to eliminate high initial catalytic activity, thus preventing excessive coke formation during initial operation. Thus for example, initial coke formation on a 321 stainless steel liner known by test to have a highly catalytic surface was substantially reduced by a one minute grit blast on each surface in a Pangborn No. 53, Type EN-Z, Blast Cleaning Cabinet with No. 120 Blastite grit. Apparently, the grit blasting exposes fresh metal surfaces which are not as highly catalytic as the original surface. Also the grit blasting produces a thin layer of cold worked metal near the surface which rapidly recrystallizes to finer grain size at the temperature of the gas generator operation. This thin surface layer rapidly reaches equilibrium with the sulfur containing carbonaceous gas products in the first minutes of gas generator operation. The advantage of grit blasting is therefore a protection of the deeper layers of the metal substrate. The use of the sulfur containing additive also inhibits the carburization and sensitization of the stainless steels used for construction of the gas generator. Thus not only the method of pretreatment of the metals but also the use of sulfur containing additive will prolong the operating life of the gas generator as well as inhibit the coke formation which is detrimental to reliable operation of the gas generator.

As previously pointed out the full passivation of the gas generator surfaces is not sufficient to eliminate the production of coke during the decomposition of ethylene oxide monopropellant therein. Thus, it is necessary that sulfur bearing additives be included in the ethylene oxide to prevent coke formation. Without additive a fully passivated reactor will soon become choked with coke because nozzle fouling and restriction begin after the first few minutes of operation. This holds true over the entire range of stay time or characteristic length (L*) over the approximate range from 300 to 2000 inch L, where long continued operation with additive is possible. There may be a very low L for example, 200 to 300 inches where additive may not be required. However, this is close to the minimum L at which a well designed reactor will give stable operation, flow rate and thrust as desired. The use of sulfur bearing compounds thus allows operation at higher L-' enabling more stable operation, easier ignition at low temperatures, and operation over a wide range in chamber pressure and L*. The maximum allowable L* for coke-free operation with the methods of this invention depends both on gas generator design and operating pressure because these affect both internal surface temperature and consequently the hot gas temperature, thus affecting the rate of coke deposition and onset of nozzle fouling. The methods of this invention, of course, extend the maximum allowable L At a reaction chamber pressure of 515 p.s.i.a. it has been found that the sulfur content of the ethylene oxide must be kept within the approximate limits of 0.2% to 0.8% by weight for low coking. Below the lower level passivation of the stainless steel surfaces of the reactor is lost and above the upper level actual interference with the decomposition and fouling of the exhaust nozzle is incurred. Within this range it is preferable that the sulfur content may be maintained at about 0.5% by weight, which is suitable over a wide range of chamber pressure and L*.

Sulfur compounds which can be used with ethylene oxide to provide the desired sulfur content are limited to those which are effective for coke prevention and compatible both with the ethylene oxide and suitable materials of construction. Of course, within this definition there are numerous sulfur bearing additives which can be successfully used. The following sulfur compounds have been proven compatible with ethylene oxide and various stainless steels in storage and are also highly effective for prevention of coke formation in the gas generator: carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl 'sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide,

1,4 dithia11e -CHg- CHgS- CHz- CH- S symmetrical trithiane C Hz-S CHI'S GETS] When symmetrical trithiane is used, it is preferred that the sulfur content be limited to low values near 0.2%.

Among the sulfur compounds just named as feasible for use with ethylene oxide, specifically carbon disulfide and thiophane (tetrahydrothiophene), although effective for coke inhibition, are definitely less preferred than the others named. It has been demonstrated that each of these two sulfur compounds leaves a noticeable carbon residue within the gas generator, so that the minimum yield of carbon residue is not kept below fifteen parts per million (15 ppm.) of total monopropellant used. Some choice may be made on the basis of certain desired physical properties such as odor, lack of odor, volatility, solubility or other minor characteristics of these sulfur containing inhibitors. However, the coke preventing effectiveness of each inhibitor named has been established by actual tests.

A large number of soluble organic compounds which contain sulfur might be deemed useful for the purpose of preventing coke formation. However, among the available sulfur bearing compounds, such additives as hydrogen sulfide, mercaptans, thiophenols, thiocresols, thioamines, thioamides, thioaldehydes, and thioacids are not compatible with ethylene oxide as they catalyze polymerization. Elemental or free sulfur is undesirable on account of lower solubility also incompatibility both with ethylene oxide and with common steel or other materials easily sulfided. Sulfur dioxide and trioxide are deleterious on account of incompatibility, if not also ineffective for coke prevention. When nitromethane is used to improve ethylene oxide ignition, the maximum acceptable sulfur content of about 0.8%, and in particular the use of dimethyl sulfide, have a unique and enhanced efiect in lowering the minimum ignition temperature. It is therefore seen that the usefulness and applicabilty of the preferred sulfur bearing compounds to be added to ethylene oxide is established only through extensive tests.

It has been explained that passivation of the gas generator and use of a sulfur compound in the ethylene oxide monopropellant increases the L* range or stay time for clean or coke-free operation of the gas generator. There are other influencing factors however. The decomposition flame temperature of the monopropellant is affected by the temperature of the ethylene oxide entering the reactor. This decomposition flame temperature in turn alters the upper L* limit for clean operation.

At a limiting high L* the residence time of the hot gases is sufficient for onset of coke formation and deposition even though the reactor is passivated and the monopropellant contains sulfur bearing additives. For successful continued operation the gas generator therefore must be operated below the maximum L limit and about which nozzle fouling commences. This maximum L* limit may, however, be altered by the use of certain other additives.

Addition of a coolant to the ethylene oxide lowers the reaction or flame temperature. In the design of an auxiliary power unit it is frequently not feasible to fully protect the fuel system from heat sources such as the gas generator, electrical components and missile shell or the normally hot atmospheric temperatures in various regions. Operation of the gas generator at high ambient temperatures raises the temperatures of the interior surfaces and the reacting gases in contact with the surfaces. Thus, a coolant may be added to the ethylene oxide monopropellant to enable operation at high fuel temperatures without increase in the reaction or flame temperature which would require a reduction in L*. The coolant which has been found particularly useful in the monopropellant disclosed in this invention is 1,4-dioxane. With ethylene oxide a 1% 1,4-dioxane addition nullifies a 15 F. rise in flame temperature which otherwise results from a 22 F. increase of fuel temperature.

The 1,4-dioxane is added to the ethylene oxide monopropellant in concentrations up to about by weight. This concentration may be varied depending upon the fuel temperatures. Suitable dioxane additions can provide the same flame temperature and the same maximum allowable L* as for fuel introduced at 77 F. For example, with a monopropellant temperature of about 160 F. a concentration of dioxane of about 4% by Weight is satisfactory to avoid reduction in L*. At about 210 F. a 6% concentration of dioxane is satisfactory. At about 250 F. 8% dioxane performs the desired function. The amount of dioxane therefore which is included in the monopropellant is computed depending upon the temperature at which the monofuel is to be introduced into the gas generator.

It is also desirable under certain conditions to lower the ignition temperature of a sulfur-bearing ethylene oxide monopropellant. The addition of nitromethane to the ethylene oxide monopropellant substantially improves the ease of ignition of the gas generator. In order to provide improved ignition characteristics in the monopropellant from about 5 to 18% by weight of the monopropellant of nitromethane may be added. However, the most preferred amount of nitromethane is about 9% by weight of the monopropellant. It is preferred that the sulfur bearing compound added to the ethylene oxide be dimethyl sulfide. It is found that this addition re- ,duces the maximum allowable characteristic length of the reactor by about 23% on account of the consequent increase in flame temperature.

The invention is further illustrated by the following examples of coke-free operation of a gas generator using ethylene oxide monopropellant.

Example 1 A cylindrical steel hot gas generator containing a stainless steel internal liner was treated for about two hours at 1500 F. by a gas mixture of 95% nitrogen and 5% hydrogen sulfide at a rate of 0.01 ft./ sec. After two hours treatment hydrogen sulfide flow was stopped, the heat source which was a muflle furnace was removed and the generator was allowed to cool while the nitrogen gas flow was continued. Thereafter a monopropellant additive mixture of ethylene oxide containing by weight 1% dimethyl sulfide which is equivalent to a 0.5% concentration of sulfur was continuously fed to the chamber at a rate of about 0.025 lbs/sec. The chamber was four inches in length and 1.75 inches in internal diameter. The resulting reaction temperature was approximately 1790 F. and the chamber operating pressure was 515 p.s.i.a. This chamber gave substantially carbon-free operation. Average carbon deposition in the chamber was reduced below p.p.m. of the feed, with no evidence of fouling of the exhaust nozzle during five hours of operation.

Example 2 To a similar reactor as passivated in Example 1 was added the ethylene oxide monopropellant of Example 1 at a temperature of about 220 F. The monopropellant also contained about 6% by weight of 1,4-dioxane to reduce the flame temperature. Substantially coke-free operation was obtained during a period of several hours of operation at high characteristic length or L* of 2220 inches. At the same inlet temperature, but without dioxane addition, this gas generator was not capable of coke-free operation above 1100 inch L*.

Example 3 A reactor as assivated in Example 1 except that the H 8 concentration was 0.4%, the nitrogen concentration was 99.6% and the flow rate was 0.4 ft./sec., was operated with an ethylene oxide monopropellant as set forth in Example 1 and containing in addition about 9% by weight nitromethane to reduce the ignition temperature. The minimum ignition temperature required was lowered more than F.

Example 4 A stainless steel decomposition chamber containing a stainless steel liner was treated by four 24 minute runs at 1710 inch L* with an ethylene oxide mixture containing 1% dimethyl sulfide and 6% dioxane. The chamber was 4 inches in length and 1.75 inches in internal diameter. This chamber when employed with a typical fuel composition as set forth in Example 1 gave substantially carbon-free operation.

Example 5 A stainless steel hot gas generator of the type in Example l, but with liner and internal surfaces coated by a ceramic enamel was operated at 2220 inch L* for 5.5 hours with ethylene oxide containing 1.6% by weight of l,4thioxane. The chamber pressure was 515 p.s.i.a. and feed rate approximately 0.025 lb./ sec. Total coke deposition in the gas generator was only 2.4 grams, corresponding to an average of 11 parts per million of the feed. No nozzle fouling was observed. However, one hour of subsequent operation with ethylene oxide containing no additive caused deposition of 6.6 grams of coke or 180 p.p.m. with noticeable fouling of the exhaust nozzle. In this case, the vitreous enamel prevented contact of the hot gases with hot metal surfaces. It was therefore established that the sulfur bearing additive inhibits both thermal coke forming reactions, as well as the coke forming reactions which are catalyzed by untreated stainless steel surfaces.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

I claim:

1. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator to achieve initial passivation by subjecting said interior surfaces to materials which convert said surfaces to a condition which when subsequently subjected to decomposition of ethylene oxide containing a sulfur bearing additive will substantially eliminate coke deposition, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

2. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator at elevated temperatures with sulfurous materials to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

3. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while sub stantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by passing an inert gas containing up to about 5% hydrogen sulfide therethrough at a temperature from about 1500 to 1700 F. to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

4. The method of claim 3 wherein said gas is passed through said generator at a rate of from 0.01 to 0.4 ft./sec. for about 2 hours and said hydrogen sulfide concentration is from about 0.4 to 5%.

5. The method of claim 4 wherein said gas flow is maintained while said generator is being cooled.

6. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by heating them for about two hours at from about 15 00 F. to 1700 F. in an atmosphere at least partially diluted with inert gas to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

'7. The method of claim 6, wherein said inert gas is a member of the class consisting of nitrogen, helium and argon and said atmosphere contains about 0.5% by weight of moisture.

8. The method of claim 7, wherein oxygen is substituted for said moisture.

9. The method of claim 7, wherein said moisture is present in a range from about 0.2 to 2.0% by weight.

10. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by initiating and carrying on the decomposition therein for limited periods of time of ethylene oxide containing by weight up to about 0.8% of sulfur added as a sulfur compound to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

11. The method of claim 10, wherein said sulfur compound is dimethyl sulfide.

12. The method of claim 11, wherein said limited periods of time total from about 1 /2 to- 2 hours.

13. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator to achieve initial passivation by initiating and carrying on a series of decompositions therein at low L*, said series being decomposition for about twenty-four minutes of ethylene oxide containing about 1.6% of dimethyl sulfide, the total duration of said series not exceeding about 2 hours, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur in the form of a sulfur bearing additive.

14. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while sub- 0 stantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator to achieve initial passivation by subjecting said interior surfaces to materials which convert said surfaces to a condition which when subsequently subjected to decomposition of ethylene oxide containing a sulfur bearing additive will substantially eliminate coke deposition, and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sufiicient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .S% in the form of a sulfur bearing additive.

15. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator at elevated temperatures with sulfurous materials to achieve initial passivation and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sufficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

16. The method of claim 15 wherein said sulfur content is about .5% by weight.

1'7. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by passing an inert gas containing up to about 5% hydrogen sulfide therethrough at a temperature from about 1500 to 1700 F. to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sufficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

18. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by initiating and carrying on the decomposition therein at low L* for a limited period of time of ethylene oxide containing up to about 0.8% of sulfur added as a sulfur compound to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sulficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

19. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by heating them for about two hours at from about 1500" F. to 1700 F. in a moist inert gas atmosphere to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sufficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

20. The method of claim 19, wherein said inert gas is a member of the class consisting of nitrogen, helium and argon and said moist atmosphere contains about 0.5% by weight of moisture.

21. A liquid monopropellant capable of substantially coke free decomposition in a pretreated stainless steel gas generator, consisting essentially of: ethylene oxide containing by weight from about .2 to .8% sulfur in the form of an effective coke preventing and compatible sulfur bearing additive.

22. A liquid monopropellant capable of substantially coke free decomposition in a pretreated stainless steel gas generator, consisting essentially of: ethylene oxide containing a sufficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

23. The monopropellant of claim 22, wherein said sulfur content is about .5% by weight.

24. A liquid monopropellant capable of use at increased fuel temperature without coking in a pretreated stainless steel gas generator, consisting essentially of: ethylene oxide having a content by weight of from about .2 to .8% sulfur in the form of a sulfur bearing additive and as a coolant up to about 10% by weight of 1,4-dioxane, depending on the fuel introduction temperature.

25. A liquid monopropellant having improved ignition capabilities when used in a pretreated stainless steel gas generator, consisting essentially of: ethylene oxide having a content by weight of from about .2 to .8% sulfur in the form of a sulfur bearing additive and from about to 18% by Weight nitromethane.

26. A method of passivating the interior surfaces of a stainless steel gas generator for operation with an ethylene oxide monopropellant having a content of a minor proportion of sulfur, comprising: pretreating the interior surfaces of said generator at elevated temperatures with sulfurous material.

27. A method of passivating the interior surfaces of a stainless steel gas generator for operation with an ethylene oxide monopropellant having a content of minor proportion of sulfur, comprising: pretreating the interior surfaces of said generator by passing an inert gas containing up to about 5% hydrogen sulfide therethrough at a temperature from about 1500 F. to 1700 F.

28. A method of passivating the interior surface of a stainless steel gas generator for operation with an ethylene oxide monopropellant having a content of a minor proportion of sulfur, comprising: pretreating the interior surfaces of said generator by heating them for about two hours at from about 1500" F. to 1700 F. in a moist argon atmosphere.

29. A method of passivating the interior surface of a stainless steel gas generator for operation with an ethylene oxide monopropellant having a content of a minor proportion of sulfur, comprising: pretreating the interior surfaces of said generator by initiating and carrying on the decomposition therein at low L* for a limited period of time of ethylene oxide containing by weight up to about 0.8% of sulfur added as a sulfur compound.

30. The method of claim 29 wherein said sulfur compound is dimethyl sulfide.

31, The method of claim 30 wherein said limited period of time is from 5 minutes to 2 hours.

32. The monopropellant of claim 25 wherein said nitromethane is present in an amount of about 9% by weight.

33. The monopropellant of claim 32 wherein said sulfur content is 0.8% and said sulfur is present in the form of dimethyl sulfide.

34. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by grit blasting to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide having a content by weight of about .2 to .8% sulfur.

35. A method of operating a stainless steel gas generator with an ethylene oxide monopropellant while substantially eliminating coke deposition, comprising the steps: pretreating the interior surfaces of said generator by grit blasting for about 1 minute to achieve initial passivation, and thereafter initiating and carrying on the decomposition therein of ethylene oxide containing a sufficient amount of a member of the class consisting of carbon disulfide, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide, diethyl sulfide, thiophane, thiophene, 1,4-thioxane, diethyl disulfide, 1,4-dithiane and symmetrical trithiane to provide a sulfur content by weight of from about .2 to .8%.

36. A method of passivating the interior surface of a stainless steel gas generator for operation with an ethylene oxide monopropellant containing a minor proportion of sulfur, comprising: pretreating the interior surfaces of said generator by grit blasting.

37. The method of claim 36 wherein said grit blasting continues for about 1 minute.

References Cited by the Examiner UNITED STATES PATENTS 2,381,257 8/1945 Campbell et a1 260-348 2,501,124 3/1950 Heath.

2,557,018 6/1951 Viles -354 2,557,020 6/1951 Viles 60-35.4 2,890,843 6/ 1959 Attinello 6035.4 2,927,850 3/1960 Forney et al. 60-354 CARL D. QUARFORTH, Primary Examiner. LEON D. ROSDOL, Examiner. 

1. A METHOD OF OPERATING A STAINLESS STEEL GAS GENERATOR WITH AN ETHYLENE OXIDE MONOPROPELLANT WHILE SUBSTANTIALLY ELIMINATING COKE DEPOSITION, COMPRISING THE STEPS: PRETREATING THE INTERIOR SURFACES OF SAID GENERATOR TO ACHIEVE INITIAL PASSIVATION BY SUBJECTING SAID INTERIOR SURFACES TO MATERIALS WHICH CONVERT SAID SURFACES TO A CONDITION WHICH WHEN SUBSEQUENTLY SUBJECTED TO DECOMPOSITION OF ETHYLENE OXIDE CONTAINING A SULFUR BEARING ADDITIVE WILL SUBSTANTIALLY ELIMINATE COKE DEPOSITION, AND THEREAFTER INITIATING AND CARRYING ON THE DECOMPOSITION THEREIN OR ETHYLENE OXIDE HAVING A CONTENT BY WEIGHT OF ABOUT .2 TO .8% SULFUR IN THE FORM OF A SULFUR BEARING ADDITIVE. 